Since the advent of the rolling cutter rock bit for earth boring, various improvements have contributed to enhanced performance of such bits in the wide variety of earth formations for which such rock bits are utilized. Among the more significant improvements particularly adapted for drilling through hard or abrasive formations has been the use of hard facing materials metallurgically applied about the teeth and cutter surfaces to render these surfaces increasingly resistant to erosion or abrasive wear. This has significantly increased the useful life of such bits and its consequent greater drilling penetration before bit replacement is required. One form of hard facing has been the use of a composite material consisting essentially of an alloy steel matrix in which are dispersed particles of sintered tungsten carbide. The composite material is applied by a welding process in which a steel tube is filled with tungsten carbide particles and ferroalloys are heated to a temperature sufficient to melt the steel tube and fuse the composite to the selected surfaces. Another form of hard facing is disclosed in U.S. Pat. No. 3,842,921 in which the bit surfaces are carburized, followed by boronizing, hardening and then tempering.
For insert type compact rock bits utilizing tungsten carbide inserts, lands are customarily provided in a circumferential row that are drilled to form sockets in which the inserts are press fitted in an interference fit. For processing carburized treated bits preliminary to drilling socket cavities, it has long been the practice to first profile the bit with circumferential lands having extra stock. The entire cone is then carburized after which the circumferential lands are machined to remove the extra stock of the lands to a depth below the carburization. Subsequently, the cone is subjected to metallurgical hardening of those portions on which carburize case has remained. When hardening has been completed cavities for the inserts are drilled into the exposed metal unhardened compact lands to receive the press fit inserts. By this means, i.e., by removing the carburized or hardened stock from the land areas, the potential for strain cracks from the force fit inserts in the relatively inelastic carburized surfaces has thereby been avoided.
As a consequence of the foregoing, the potential for cracking has been eliminated while the majority of the exposed cone surface is case hardened from the applied carburizing. However, the space between adjacent inserts in the common circumferential row from which the carburizing has been removed along with removal of the extra stock is relatively non-hardened and subject to greater erosion from cuttings while drilling than are the more hardened surfaces. The latter is obviously a weak link in the overall structure and detrimental to the cutter as a whole in wanting to preserve the integrity of the cutter unit for maximized earth penetration. Yet despite recognition of the problem, a ready solution therefor has not heretofore been known.